Ladders or stairs employed with a cylindrical tank or other cylindrical structure are often affixed to the tank wall and are disposed along a helical path along the tank wall. The ladders are constructed in conventional manner with treads of sufficient width to accommodate both feet of a user standing upon the tread, the treads being supported by stringers extending along the ladder way. In ascending or descending a ladder, the right foot of a user steps upon the right-hand portion of a tread, and the left foot steps upon the left-hand portion of the next tread. Thus, alternate treads are employed by each foot of the user, and it is not usually necessary to stand upon a single tread with both feet during ascent or descent of a ladder. Since only half of a tread is being used, the presence of a full-width tread in a conventional ladder is wasteful of material and adds to the weight of the overall ladder structure. The presence of a full-width tread also presents an obstacle to the movement of a user's foot in ascending or descending the ladder, creating an unnecessary safety hazard for the user. The inclination of a conventional ladder must be such that a user can move his foot past the unused portion of a tread in moving to the next tread, thereby limiting the angle of inclination.
In U.S. Pat. No. 4,199,040, issued Apr. 22, 1980, entitled Ship Ladder, and assigned to the same assignee as this invention, an improved ladder is disclosed which comprises a single stringer or central tread support disposed between upper and lower levels at a predetermined angle of inclination and having an array of half treads on each side of the stringer, each array being vertically spaced from the other along the length of the stringer. The half treads are affixed to, and laterally extend from, the respective sides of the stringer, and each includes an integral outwardly extending portion which terminates in a plane which is forward of a plane passing through the front edges of the treads. First and second handrails are disposed in this forward plane and are affixed to and supported by the outwardly extending tread portions. This novel ladder can be disposed at a relatively steep angle in comparison to a conventional ladder of the same tread width and riser height and provides sufficient safety and comfort to permit balanced use of the ladder even without holding onto the handrails.
Ladders are known in which treads or rungs are alternately arranged along a single stringer or pole. In ascending and descending ladders of this known type, a user must face the ladder and support himself by holding onto the rungs to guide his ascent or descent. Such ladders of known construction cannot be readily descended facing forward, as with a stairway, and these known ladders also require a fair degree of dexterity on the part of a user and are not very comfortable to use.